Add a 'set' lattice backed by maps

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

Lattice/Unit.agda

@@ -3,7 +3,7 @@ module Lattice.Unit where
 open import Data.Empty using (⊥-elim)
 open import Data.Product using (_,_)
 open import Data.Nat using (ℕ; _≤_; z≤n)
-open import Data.Unit using (⊤; tt)
+open import Data.Unit using (⊤; tt) public
 open import Data.Unit.Properties using (_≟_; ≡-setoid)
 open import Relation.Binary using (Setoid)
 open import Relation.Binary.PropositionalEquality as Eq using (_≡_)